Batch fabrication of very small devices, such as electronics, sensors, and actuators, can be based on wafer handling systems, where the wafer functions as a substrate onto which the device is built during the production steps. The production of such devices is mostly done using a stacking method. The device is built by stacking layer on layer. Each layer can be of a different material, have a different thickness, and can be continuous over the device or laterally confined. The lateral shape of the layer can be defined by including a patterning step based on standard photolithography and microfabrication technologies. Common microfabrication processing steps are for instance ion diffusion, oxidation, deposition of material using e.g. spincoating, physical and chemical deposition methods, removal of materials using e.g. wet and dry etching methods. For an overview of such methods, reference is made to G. T. A. Kovacs, K. Petersen, and M. Albin, “Silicon micromachining; Sensors to systems”, Analytical chemistry, 1996, 68, p. 407A-412A. In the last step the device can be removed from the substrate. Common methods are dicing and sacrificial layer. A method called “differential adhesion” is also used.
In dicing, the device, such as integrated circuits (computer chips) that has been built on a silicon wafer, is subsequently cut out using a saw. In this case the substrate becomes a part of the product. This can be a drawback, since the properties suitable for a material to hold a device during process not necessarily go together with the properties required for the final device.
The sacrificial layer method uses an intermediate, sacrificial layer between the substrate and device. The device is built up onto the sacrificial layer. The device is then released from the substrate by removing the sacrificial layer, e.g. by dissolving the layer using an appropriate etchant or solvent. These etchants or solvents can damage the layers of the device. Also, the removal process can take hours for large area devices.
The differential adhesion method is based on the poor adhesion between the top layer of the substrate and the bottom layer of the device. The device is built up using the above mentioned microfabrication steps. As a last step in the production the device is removed from the substrate, by “peeling” it loose. The technique of differential adhesion is described in U.S. Pat. No. 6,103,399.
All of the above production methods however have the disadvantage that these processes are bottom up processes. Only one side is available for processing. Layers are added on top of each other on one side of the device. This means that sensible materials that are added on the bottom side of the devices might be damaged by chemicals during the process. Due to the device design, these layers cannot be added at a later stage in the process, as this means that they will be on the topside of the device. Inversion of the process steps, i.e. starting with the “top layers” on the substrate is not always possible either. Which, for instance, is the case for processes such as electroplating and electropolymerisation where an electrically conducting seed layer is needed. The device can only be built as substrate/seed layer/electoplated layer and not vice versa.
It is therefore an object of the invention to solve the above-described problems.